1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (resist). Instead of a mask, the patterning means may comprise an array of individually controllable elements that generate the circuit pattern on an impinging light beam.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one pass, and scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
In order to manufacture devices using lithographic techniques, it is typically necessary to form the device from multiple layers. When producing such a device from multiple layers, as each layer is created it is aligned with the previous layers. It has therefore been known to provide alignment marks on a substrate. Before each layer is exposed on the substrate, it is transported to an alignment measuring area, where the alignment marks are detected, allowing a precise determination of the position of the substrate relative to the alignment sensors. By moving the substrate in a controlled manner to the exposure position, a positional correction can be applied to accurately produce the subsequent layer in the correct position on the substrate. Such a system can be used to ensure that the overlay errors are small in comparison to the critical feature size.
However, as the critical feature size continues to diminish, further improvements in the overlay accuracy are required. Furthermore, as the alignment requirements increase, the time taken to locate and inspect the alignment marks increases, reducing the throughput of the apparatus.
Therefore, what is needed is a method and an apparatus in which the overlay accuracy can be improved without significant loss of throughput of the apparatus.